The present invention relates generally to nuclear reactors and, more particularly, to removing and installing control rod drives (CRDs) in a nuclear reactor.
Control rod drives are used in a nuclear reactor to control the position of the control rods within the reactor core. The CRDs typically extend from the reactor pressure vessel in the under vessel area.
Occasionally CRDs are removed for maintenance and repair. Subsequent to servicing, the CRDs are reinstalled. CRDs should be moved in a safe and controlled manner to prevent damage to the CRD and/or adjacent equipment. After removal from a housing, the CRD should be rotated from a vertical position to a horizontal position, and secured for transporting out of the tinder vessel area. These activities should be performed by a minimum number of personnel and accomplished quickly and efficiently to reduce radiation exposure.
One known method for exchanging a CRD is to attach an extension tube to the bottom of the CRD and thread a cable over a stationary idler pulley and through the bottom of the extension tube. The cable is secured to a static hanger. A winch then pays out the cable and the CRD is lowered until the extension tube contacts the floor of the under vessel area. A second cable winch is then employed to secure a clamp around the CRD at the approximate mid-length of the CRD. With the second winch securing the weight of the CRD, an operator is required to enter the basement of the under vessel area to remove the extension tube. The second cable winch then pays out cable until the CRD is out of the housing. Once the CRD is out of the housing, the CRD is then manually positioned from the vertical position to a horizontal position as the second winch lifts the CRD up to the equipment platform. When the CRD is above the lower track, or rail, on the equipment platform, a transport cart is rolled into position under the horizontal CRD. The second winch then pays out the cable to land the CRD on the cart. The cable rigging is removed from the CRD, and the cart with the CRD is pulled out of the under vessel area.
Another known system for removing a CRD is a rigid tower system, which allows the CRD to be lowered into a tower supported by a trunnion cart. The CRD is raised and lowered by an elevator secured within the tower. The known tower systems use either roller chains, timing belts, cable or hydraulic cylinders, and employ built-in air or hydraulic motors with supply and control (air) logic and hydraulic lines to raise and lower the elevator tower. The known tower systems also incorporate a separate extension tube, which provides the extended throw required to install or remove a CRD.
There are a few drawbacks with the currently known systems for exchanging CRDs. The currently known systems tend to be complex, time consuming to operate, and are susceptible to causing a CRD to fall onto the floor of the under vessel area. The currently known systems may expose operators to higher levels of radiation. The currently known systems may include built-in drive motors, which may be difficult and time consuming to replace if failure occurs during CRD exchange. The currently known systems may include components subject to damage and failure causing down time and high maintenance costs. The currently known systems may include two parallel screws synchronized with a timing belt to move the CRD, wherein the timing belt may slip causing the elevator to become unleveled; thereby preventing movement of the CRD.
For the foregoing reasons, there is a need for a system for exchanging a CRD. The system should include two independent screws with independent travel. The system should not include: roller chains, timing belts, cable, or hydraulic cylinders. The system should not employ built-in air or hydraulic motors with supply and control (air) logic and hydraulic lines to raise and lower the elevator tower. The system should not require a separate extension tube.